Abstract: A tubus for defining the boundary of a beam of penetrating rays, which is preferably adapted for use in the radiation therapy. In a tubus of the above-mentioned type, the tube wall is thereby constructed of a plurality of over-lapping wall elements directed in parallel with the edge radiation of the radiation cone encompassed by the tubus, and suspended on the tubus housing so as to be displaceable to the direction of the edge radiation. That type of construction of the tubus facilitates the cross-section and also the surface of the focused radiation cone to be varied. Thereby it becomes possible to correlate the radiation field in a stepless manner by means of one and the same tubus over a wide range to almost every form and size of the illness spread.